The present invention relates to lifesaving vehicles for ships and offshore platforms, and in particular to such vehicles that free fall to the water and decelerate from water resistance in a controlled manner.
Escape vehicles used to leave stricken ships and offshore platforms include open lifeboats and enclosed escape capsules.
Typically, lifeboats and escape capsules mount on davits well above water level. Winches control their descent from a stricken vessel or offshore platform to the water. People enter the escape vessels, winches operate to lower the vessels from the stricken ship or platform to the water, and the vessels are released from the winches.
These traditional launching processes and mechanisms have many disadvantages: Quite often it is difficult to operate the davits and winches. The process is time consuming, and time can be critical. The launch mechanism is comparatively complex because of winches and attendant machinery. With this complexity there is possibility in heavy seas that waves will make launching difficult or unsuccessful; for example, waves could prevent proper release of the escape vehicle from the launching mechanism. The davit method places the escape vehicle close to the ship or platform being evacuated, and that can be dangerous because of vessel or platform interference with the path of the lifeboat or escape capsule.
Free falling escape vehicles offer significant advantages over traditional vehicles. They are simple, easy to operate, and can be made to launch free of the ship or platform with a simple launching apparatus. A free falling vehicle may be mounted so that gravity alone effects launch with the release of a restraint. In some instances, however, it may be satisfactory to use davit and winches to launch free falling escape vehicles by lowering the vehicles part of the way and then the vehicles are released to free fall the rest of the way to the water.
Movement away from the ship or platform upon release can be done by pendulously supporting the vehicle to produce a horizontal component of motion upon release.
Free falling escape vehicles, however, must provide controlled deceleration upon entering the water, for the distance that they fall can be considerable, as much as 30 meters for an ocean platform not being an uncommon requirement. The escape vehicle and its occupants falling from even a modest distance without controlled deceleration would be worthless: the occupants would suffer unacceptable injury, the vehicle could be structurally damaged and not seaworthy.
Acceptable rates of deceleration of an escape vehicle entering the water is from between about 4 g to about 10 g. It is also desirable to minimize jerk, that is, the time rate-of-change of acceleration, and so uniform deceleration should occur to the extent practical.
An escape vehicle must be dynamically and statically stable in the water over a broad range of entry angles so that the attitude of the vessel will be correct after the effects of impact with the water are over. A vehicle that because of the angle at which it hits the water rolls over on its side would be totally unacceptable. The angle of entry can be effected by uncontrollable variables such as the state of the sea or the angular orientation of the ship or platform, for example. The vehicle then, must be tolerant of water entry angles. It must have dynamic stability and be able to maintain its correct attitude during the deceleration process; it must have static stability to be able to maintain its correct attitude after the effects of deceleration are gone.